When a command is inputted, a guidance display of a conventional interactive system displays the inputted command name, what parameters are required next by the command, a value of an inputted parameter and the command input data that have so far been inputted in the form of a guidance of character strings. When an undo processing of the command is executed, the command processing that is executed last becomes an object of display, but the object of the undo processing is not clearly displayed for the operator. A conventional technique of this kind is disclosed in the chapter 5.4 "Undoing of Inputted Command" in "Manual of HITAC Programming Supported Editor ASPEN", pp. 92.
Furthermore, "VLSI Silicon Compilation and The ART of Automatic Microchip Design", published by Prentice-Hall, Inc., 1983, discloses a method of describing geometric points in terms of the two Y-values, BOTTOM and TOP, on page 26 with reference to FIG. 1.12 as a display system of a parametric figure. This system describes the apex coordinates of a figure only by characters by use of constants, variables and their calculation formulas.
When an operator wants to know the commands that have been executed in the past, he must read the necessary information from the history of the commands displayed as character strings, which requires a long time for understanding. When the number of kinds of commands becomes great, he is likely to make a mistake because there exist similar command names and because the number of characters of the command name becomes longer. Furthermore, the operator must interpret the relation between a command and a figure prepared by the command from the parameters of the command displayed on a display surface. This interpretation is difficult even for the command that is executed last and is extremely difficult particularly when the past commands and the command history are not displayed. When a command for undo processing is made, the operator is likely to misjudge which figures are affected by the undo processing and to execute unnecessary undo processing. Accordingly, there has been such a drawback that erroneous operations of the graphic display apparatus are likely to occur frequently.
The conventional parametric graphic display system explained above is not easy to comprehend by the operator and therefore involves the following problems.
Since the system represents a given figure only by characters, the system is not visual and it is extremely difficult for the operator to understand the positioned relation between apexes and the shape of the figure and to form an image of the actual figure in his or her mind.
Since the parametric figure as the object is expressed by use of the coordinates of the apex, it is necessary to express all the coordinates of the apexes that move when the distance is made variable, even though the distance between points (or sides) which is to be made variable is only one. Accordingly, the formulas representing the apex coordinates get complicated with the increase in the number of parameters, a great deal of time and labor are necessary to express a parametric figure and mistakes are more likely to occur.
As described above, the conventional method requires a long time for the preparation and correction of a parametric figure.